The study of experimentally induced mammary tumors has focused primarily on several mouse strains that are infected with mouse mammary tumor virus (MMTV) and have been inbred for a high incidence of mammary tumors. We have expanded this study to include other species of MMTV-infected feral mouse strains that have not been bred for high mammary tumor incidence. MMTV appears to induce mammary tumors by acting as an insertional mutagen that leads to the activation of previously silent cellular genes (int genes). We have previously shown that the inbreeding program used to develop the high-incidence mouse strains may also have fixed other cellular mutations that affect the frequency with which int-1 and int-2 are activated. The int-1 and int-2 genes are much less frequently activated in feral mouse mammary tumors, raising the possibility that other int loci are active in these tumors. We described one such locus, designated int-3, in CZECHII V+ feral mice. The organization of int-3 differs from that of int-1 or int-2. MMTV insertions occur within a 500-bp region of the cellular genome on chromosome 17. Viral insertion at this locus leads to the activation of the expression of two cellular RNA species that correspond to host flanking sequences located 5' and 3' of the viral insertion site, respectively. We are currently determining the nucleotide sequence of recombinant cDNA clones corresponding to each of these RNA species. Mice chronically infected with MMTV frequently contain preneoplastic hyperplastic alveolar nodules (HAN), which can be transplanted as hyperplastic outgrowth (HOG) lines. We have found that in three of four CZECHII HOG lines, int-1 is occupied by an MMTV genome. This suggests that activation of int-1 is an early event in MMTV-induced mammary tumorigenesis. Human breast tumors frequently express tumor-associated antigens. One of these, the carcinoembryonic antigen (CEA) is a member of the family of related glycoproteins that are expressed normally in fetal tissue, the placenta, and other adult tissues. We have found that MMTV-induced CZECHII V+ mammary tumors express a 4.0-kb RNA species that is related to human CEA.